This disclosure relates to the evaluation of underground formations penetrated by a wellbore. More particularly, this disclosure relates to methods and apparatuses for facilitating the injection of fluids into an underground formation and for monitoring the directions in which the injected fluids flow within the formation and displace the formation connate fluids.
In the evaluation of reservoirs, it is desirable to understand, measure, and test how fluids move through the formation. A number of methods are currently used to test reservoir fluid mobility and formation permeability and relative permeabilities. Some of these techniques include the measurement of invasion by a drilling fluid. Other techniques are generally known as formation testing and core analysis.
A determination of drilling fluid invasion can be a useful measure indicative of an approximate permeability of the formation. However, this approach may be limited by an insufficient invasion process, in particular due to the creation of a mud cake. Additionally, the permeability measured from invasion is related to the relative permeabilities of the mud filtrate and the connate formation fluid. The permeability measured from invasion may provide little indication of the relative permeability curves when fluids other than the mud filtrate displace the connate formation fluid. Further, it is assumed that the invasion process is uniform around the wellbore and therefore, the permeabilities derived from this analysis do not take into account the formation anisotropy.
Formation testers can determine in-situ reservoir fluid mobility in response to a drawdown, but formation testers typically cannot inject fluids into a reservoir due to the presence of a mud cake. In some cases, pumping fluid from the formation may be sufficient to remove the mud cake. However, in many cases, pumping fluid from the formation may not produce a high enough flow velocity to reliably remove the entirety of the external mud cake from the wellbore wall and the internal mud cake which occupies the pore space just beyond the wellbore wall. During injection, the residual mud cake and mud particles (including drilling fines) may re-seal the wellbore wall and thus may limit or prevent further fluid injection. Thus, in many cases, injecting fluid into the formation may not be possible in an open hole environment. Further, the presence of mud cake, particles and formation damage at the near-wellbore sand face can significantly interfere with the fluid mobility observed by the formation tester. Still further, increasing the flowing pressure induced by the formation tester in such an environment will typically result in a loss of seal of the formation tester against the wellbore wall or may induce a fracture in the formation. If the seal is lost, the formation tester will no longer be in hydraulic communication with the reservoir formation and any measurements will not be representative of the reservoir formation. Once a fracture has been created in the reservoir formation, subsequent mobility or permeability measurements may be dominated by flow into and out of the fracture and thus will not be representative of the reservoir formation.
When analyzing a core for determining formation relative permeabilities, a sample of formation rock is cut, brought to surface and its properties are tested in a laboratory. However, it can sometimes be difficult to recreate in the surface laboratory the representative downhole conditions, such as pressure, temperature and fluid properties.
Systems for injecting fluids into formations do exist today. For example, the mud cake may be dissolved or flushed away with a chemical solvent such as an acid. However, mud cake solvents are typically highly corrosive. These solvents may present a safety hazard to operational personnel and may damage some of the components of a formation tester. Therefore, these injection systems usually require the mud in the wellbore to be replaced with a completion fluid and the mud cake to be dissolved using acids. In some cases, this requires at least a portion of the well to be cased, perforated, and completion equipment such as tubing and packers to be installed before injection can be performed. In these cases, measurements derived from injection in the reservoir formation may come too late to make critical decisions regarding the well completion. Also, the zones which can be injected into may be limited by the locations of the perforations. Further, the presence of casing during the injection may limit the type of downhole measurement tools which can be used to monitor the injection front to those downhole measurement tools that can perform measurements into the formation through a casing (usually metallic, magnetic and conductive) and are suitable to a cased hole environment.